Semi-global shutter imager

ABSTRACT

This disclosure is directed to an image sensor. The image sensor includes a two-dimensional pixel array divided into a plurality of blocks, each of the plurality of blocks comprising pixels arranged in at least two different rows and two different columns, and a shutter mechanism that exposes the plurality of blocks sequentially, with all pixels in each block being exposed synchronously.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/163,730, filed on May 19, 2015, entitled “SEMI-GLOBAL SHUTTERIMAGER”, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present disclosure relates to the field of digital imaging and inparticular to a semi-global shutter imager that can capture multiplepixel blocks sequentially but with the pixels in each block capturedsynchronously.

BACKGROUND OF THE INVENTION

An image sensor (or imager) generally refers to the part of animage-capturing device (e.g., a camera) that can detect and convey theinformation required to form an image. In a digital camera, the imagesensor can typically be a silicon semiconductor on which images arecaptured. Structurally, the sensor can be composed of an array ofphotosensitive diodes (i.e., photosites) that capture photons andconverts them to electrons. The buildup of electrons in each photositecan be converted to an electronic signal (e.g., a voltage), which can inturn be converted to digital data representing a picture element orpixel. These elements or pixels can then be used for assembling thefinal image. The final image can be stored in the camera's memory andretrieved to be viewed on a display and/or further manipulated.

Ideally, a well-designed image sensor allows the camera to have arelatively high-speed frame rate that enables the camera to capture alarger number of images within a short period of time and, at the sametime, uphold image quality by minimizing the amount of undesirableeffects such as motion artifacts in the result images.

BRIEF SUMMARY OF THE INVENTION

This disclosure generally relates to a semi-global shutter imager andmechanism that can capture and process multiple pixel blockssequentially with the pixels in each block captured synchronously. Thesensor elements (or pixels) of the image sensor can be divided intomultiple pixel blocks. All pixels in the same block can be exposed tolight at the same time. Thereafter, while data from the exposed block ofpixels is still being read out, another block of pixels can be exposed.This process can repeat until all pixels are exposed and read out. Thiscan significantly reduce the delay between the exposure of the pixelsand when the information captured in the pixels are analyzed, therebyincreasing the frame rate (or speed) or the camera. In addition, bysetting an optimal number of pixel blocks for a given semi-globalshutter image sensor and exposing all pixels in each pixel block at thesame time, the semi-global imager can also reduce certain forms ofundesirable distortions (e.g., motion artifacts) to the result imagesthat can be caused by the sequential exposures of the pixel blocks.

Generally, an image sensor may include a two-dimensional pixel arraydivided into a plurality of blocks, each of the plurality of blockscomprising pixels arranged in at least two different rows and twodifferent columns, and a shutter that exposes the plurality of blockssequentially, with all pixels in each block being exposed synchronously.The shutter may, for instance, be electronically controlled. The pixelarray may have various configurations. For example, the pixel array mayinclude a rectangular array with M rows of pixels where M is no lessthan 100, and where a height of each block is at least one twentieth ofa combined height of M rows but no more than one fifth of the combinedheight of M rows. As another example, the pixel array may include arectangular array with N columns where N is no less than 100, andwherein a width of each block is at least one twentieth of a combinedwidth of N columns of pixels but no more than one fifth of the combinedwidth of N columns. Some of the blocks may include different number ofpixels in other suitable arrangements and geometric shapes, while someof the blocks may include the same number of pixels.

In some variations, the image sensor may include or be configured foruse with a timing control module that transmits a timing signal to eachof the blocks, where the timing signal initiates a sequence of exposuresof the blocks. Furthermore, the image sensor may include or beconfigured for use with separate readout electronics for each of theplurality of blocks, where the readout electronics may be capable ofreceiving and processing electronic signals from the pixels in acorresponding block. Such separate readout electronics may, forinstance, include an amplifier that amplifies the electronic signals andan analog-to-digital converter that converts the electronic signal todigital data. Furthermore, in one variations of the image sensor, thereadout electronics may read out electronic signals from a first blockof pixels immediately after an exposure of the first block is completed,and an exposure of a second block begins before the readout of theelectronic signals from the first block is completed. In anothervariation, there may be delay between the exposure of a second block andthe exposure of a first block, where the delay is great enough to allowfor a readout of the first block before the second block completes itsexposure.

Generally, a method of capturing an image with an image sensor mayinclude dividing a two-dimensional pixel array image area of the imagesensor into a plurality of blocks, each of the plurality of blockscomprising pixels arranged in at least two different rows and twodifferent columns, and sequentially exposing the plurality of blocks,with all pixels in each block being exposed synchronously. The methodmay be used with image sensors of various configurations, such as animage sensor with a two-dimensional pixel array including M rows ofpixels, where M is no less than 100, and where a height of each block isat least one twentieth of a combined height of M rows of pixels, but nomore than one fifth of the combined height of M rows. As anotherexample, the method may be used with an image sensor with atwo-dimensional pixel array including N columns, where N is no less than100 and a width of each block is at least one twentieth of a combinedwidth of N columns of pixels but no more than one fifth of the combinedwidth of N columns.

The method may include transmitting a timing signal to each of theblocks, where the timing signal initiates a sequence of exposures of theblocks. The method may also include reading out electronic signalsarranged in at least a first block and a second block of the pluralityof blocks sequentially. In one variation, the electronic signals fromthe first block of pixels may be read out immediately after an exposureof the first block is completed, and an exposure of the second block maybegin before the readout of the electronic signals from the first blockis completed. Furthermore, there may be a delay between the exposure ofthe second block and the exposure of the first block where the delay isgreat enough to allow for a readout of the first block before theexposure of the second block is completed.

Generally, a digital camera may include an image sensor including atwo-dimensional pixel array divided into a plurality of blocks, each ofthe plurality of blocks including pixels arranged in at least twodifferent rows and at least two different columns; a lens that directslight to the image sensor; a shutter that exposes the plurality ofblocks sequentially, with all pixels in each block being exposedsynchronously, and a timing control module that controls the timing of asequence of exposures of the blocks; readout electronics for each of theplurality of blocks where the readout electronics are capable ofreceiving and/or processing electronic signals from the pixels in acorresponding block; and a camera application-specific integratedcircuit (ASIC) that assembles an image from an output of the readoutelectronics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the exemplary components of adigital camera 100, according to an embodiment of the disclosure.

FIG. 2a is a block diagram illustrating the exemplary components of aCCD image sensor 200 with a global shutter.

FIG. 2b is a block diagram illustrating the exemplary components of aCMOS image sensor 210 with a rolling shutter.

FIG. 3 a block diagram illustrating the exemplary components of an imagesensor 300 with a semi-global shutter, according to an embodiment of thedisclosure.

FIG. 4 illustrates an exemplary timeline of the exposures and readoutsof three of the blocks in the semi-global shutter imager of FIG. 3,according to an embodiment of the disclosure.

FIGS. 5a-5c illustrate various pixel block configurations suitable forvarious semi-global shutter imagers, according to an embodiment of thedisclosure.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of preferred embodiments, reference is madeto the accompanying drawings which form a part hereof, and in which itis shown by way of illustration specific embodiments which can bepracticed. It is to be understood that other embodiments can be used andstructural changes can be made without departing from the scope of theembodiments of this disclosure.

As used herein, the term “digital camera” can refer to any digitalimage/video capturing device with an image sensor. The terms “imagesensor” and “imager” can be used interchangeably to describe one or morecomponents in a digital camera that can detect and convey theinformation that forms one or more images. The image sensor can includean image area composed of an array of sensor elements such asphotosites. The terms “photosite,” “photosensitive diode,” and“photodiode” are used interchangeably in this document. Each sensorelement can also be referred to as a “pixel” of the image sensor. Theterms “block” and “pixel block” can refer to a region in the image areathat includes multiple pixels of the image sensor. It should be notedthat each block of pixels can be of any size and geometric shape,according to the embodiments of this disclosure. In the preferredembodiments, the blocks can be contiguous and/or have the largestpractical ratio of area to boundary. The terms “image,” “result image,”and “final image” can be used interchangeably to refer to a digitalimage captured by the image sensor of a digital camera. An image can bea standalone image or a frame of a video.

This disclosure generally relates to a semi-global shutter imager andmechanism that can capture and process multiple pixel blockssequentially with the pixels in each block captured synchronously. Thesensor elements (or pixels) of the image sensor can be divided up (orgrouped) into multiple pixel blocks. All pixels in the same block can beexposed to light at the same time. The exposure can be achieved by anysuitable electrical control over the integrating element. For example,it can involve a combination of tying the charge surface to ground andclosing a CMOS transistor from the element to the readout latch.Thereafter, while data from the exposed block of pixels is still beingread out, another block of pixels can be exposed. This process canrepeat until all pixels are exposed and read out. This can significantlyreduce the delay between the exposure of the pixels and when theinformation captured in the pixels are analyzed. In addition, by settingan optimal number of pixel blocks for a given image sensor and exposingall pixels in each pixel block at the same time, the semi-global imagercan also reduce certain forms of undesirable distortions (e.g., motionartifacts) to the result images that can be caused by the sequentialexposures of the pixel blocks.

FIG. 1 is a block diagram illustrating the exemplary components of adigital camera 100, according to an embodiment of the disclosure. Thedigital camera 100 can include a lens 102, an image sensor 104, anAnalog-to-Digital (A/D) Converter 106, a camera Application-SpecificIntegrated Circuit (ASIC) 108, a storage device 110, and one or moreinput/output (I/O) devices 112. When a shutter button (not shown inFIG. 1) on the camera 100 is pressed, light can pass through the lens102 and reach the image sensor 104. The image sensor 104 can include atwo-dimensional array of sensor elements (e.g., photosites) 114 capableof capturing the light. In certain types of image sensors (e.g., CCDsensors), an electric change can build up in each photosite based on theamount of light captured. The accumulated electric charge can then betransformed into a digital signal (e.g., a digital number) by the A/DConverter 106. In other types of image sensors (e.g., CMOS sensors),each photosite can read out how much light is hitting the pixel at themoment of exposure and convert that into an electronic signal withoutstoring any charge. The electronic signal can then be digitized by theanalog/digital (A/D) Converter 106 and then processed by the cameraapplication-specific integrated circuit (ASIC) 108 to form an imageelement.

A final image can be assembled when the camera ASIC 108 finishesprocessing the digital signals embodying the data captured by each andevery photosite 114 of the image sensor 104. The process of forming theimage can include the optional step of removing background noise fromthe result image. The image can be stored in the storage 110 and/oroutput via the I/O device 112. The storage device 110 can be anysuitable storage device including but not limited to a memory card, harddrive, internal memory, and external storage space such as a cloudstorage service. The I/O device 112 can output the image to a display(not shown in FIG. 1) of the digital camera 100 or transmit it over anetwork to another device.

It should be understood that FIG. 1 only illustrates some of theexemplary components of the digital camera 100, which may include othercomponents not shown in FIG. 1. For example, the digital camera 100 canalso include a flash, zoom and focus modules, anti-aliasing filter,battery, and other components commonly found in a digital camera. Thecamera ASIC 108 can be connected to a central processing unit (CPU)designed to perform operations of the digital camera that are nothandled by the camera ASIC 108.

Embodiments of the present disclosure are generally directed to theimage sensor 104 illustrated in FIG. 1 and the shutter mechanismsassociated with the image sensor 104. The shutter mechanism in a digitalcamera 100 can control the exposure of the sensor array of the imagesensor 104 when an image is taken by the camera 100. The shuttermechanism can be electronic, mechanical, or a combination of both. Atiming mechanism can be built in the camera to control light exposuretime for the pixels of the image sensor and, depending on the shuttermechanism, the order in which the pixels are exposed. Presently, mostdigital cameras employ one of two types of shutter mechanisms, either aglobal shutter or a rolling shutter. As described in the followingparagraphs, these two types of shutter mechanisms have their respectiveadvantages and drawbacks.

Global shutters can typically be found in cameras with Charge CoupledDevice (CCD) image sensors. FIG. 2a is a simplified block diagramillustrating the exemplary components of a CCD image sensor 200 with aglobal shutter. For illustration purposes, the CCD image sensor 200 isshown to include a 4×4 two-dimensional pixel array 202, although itshould be understood that the image sensor can include any number ofcolumns and rows of pixels aligned in any configuration, or pixelsarranged in any suitable configuration. In fact, it is not uncommon fora CCD image sense to have hundreds or even thousands of rows and/orcolumns of pixels. Each pixel can correspond to a photosite of the imagesensor 200 and be capable of capturing photons. The pixels 202 can beconnected to an electron transfer register 204, which can in turn beconnected to readout electronics 206 that can include, but are notlimited to, an amplifier and A/D converter. It should also be understoodthat the CCD image sensor 200 can include other components that areomitted from FIG. 2a for clarity purposes.

The global shutter of the imager 200 of FIG. 2a can operate such thatwhen the shutter is “open,” light can reach the entire sensor array ofthe imager 200 for a predetermined period of time (shutter time orexposure time). During that time, all pixels 202 of the image sensor 200can capture light (e.g., accumulate charge) in parallel. When theshutter is blocked, light can no longer reach the sensor and the pixelscan become inactive. The global shutter in a CCD digital camera can beelectronically controlled in order to control the exposure.

After the capture is completed, the signals collected in the pixels canbe transferred, for example, one pixel at a time, to the electrontransfer register 204, from where they can be read out and processed bythe readout electronics 206. The process can include amplifying thesignals and/or converting them into a digital format (e.g., digitalnumbers), which can form the result image.

In digital imaging, the result image can include a number of keypoints,which can be defined by a small section of the image (e.g., a 5×5 or10×10 pixel block) that includes one or more highly localizable andrecognizable features of the image. The keypoints can be produced byprocessing one or more of the raw pixels. Each keypoint can include adescriptor that describes the keypoint so that it can be recognized inthe different frames of a series of images taken consecutively. Themovement of the camera and the direction in which it is pointing can bedetermined by matching the various keypoints from the different frames.This approach is often used when implementing SLAM (simultaneouslocalization and mapping) algorithms.

This sequential charge-transfer approach to the electron transferregister 204 and the readout electronics 206 employed by a globalshutter imager can be time-consuming, especially if the image sensorincludes a large number of pixels, because the data from each pixel ofthe imager has to be read out sequentially. That can mean that the delaybetween the exposure of the last pixels (along with all other pixels ina global shutter imager) and when these pixels can be analyzed (i.e.,after all other pixels have been analyzed) can be significant. In otherwords, when information read out from these last pixels is finallyprocessed, it may not necessarily reflect the current position of thecamera with respect to a scene accurately if there was relative movementbetween the camera and the scene. For example, if it takes a typicalglobal shutter CCD imager around 15 milliseconds between exposures toread out all the pixel values and calculate the keypoints of the image,there can be a 15 milliseconds delay before information from the lastpixels are analyzed, which may affect, for example, the determination ofthe camera's movement (or movement in the scene being captured). This isone of the main drawbacks of a global shutter imager.

Because a global shutter allows a CCD image sensor to capture an entireimage at the exact same moment by exposing all pixels simultaneously,one of the advantages of the global shutter is that the result image canusually have relatively high quality and be free of significantundesirable effects such as motion artifacts. All the keypoints (e.g.,small pixel blocks) can be kept intact and easily identifiable in aseries of consecutive images.

A second type of shutter mechanism, the rolling shutter, can typicallybe found in digital cameras with complementary metal-oxide-semiconductor(CMOS) image sensors. In contrast to the global shutter, a rollingshutter can allow individual pixels or individual columns/rows of pixelsto be exposed sequentially so that the information at a first pixel (ora first column/row of pixels) can be read out while the next pixel (ornext column/row of pixels) is being exposed to light. The CMOS imagesensors can achieve this rolling effect by turning on/off the pixels (orcolumns/rows of pixels) systematically in a predetermined order.

FIG. 2b is a block diagram illustrating the exemplary components of aCMOS image sensor 210 with a rolling shutter. For illustration purposes,the CMOS image sensor 210 is shown to include a 4×4 two-dimensionalpixel array, although it should be understood that the image sensor canhave any number of columns and rows of pixels aligned in anyconfiguration, or arranged in any suitable configuration. Each pixel211, 212, 213 of the two-dimensional pixel array of FIG. 2b can be aphotodiode capable of converting light into electronic signals. A gridof conductive interconnects 214, 218 overlaying the image sensor 210 canconnect the pixels by rows and columns for applying timing and readoutsignals. In particular, the interconnects can include row signal lines(collectively 214) for transmitting timing signals from a clock andtiming control module 216 to each pixel (or each row/column of pixels).The timing signal can control when each individual pixel or row ofpixels is exposed to light and also when the electronic signals fromeach pixel are read out. The interconnects can also include verticaloutput lines (collectively as 218) for reading out the signals from thepixels and transmitting them to readout electronics (collectively as220) for further processing. As illustrated in FIG. 2b , each outputline can be connected to separate readout electronics 220. The readoutelectronics 220 for each output line can include, for example, anamplifier and A/D converter, which can amplify and convert the signalsinto a digital format (e.g., digital numbers). The digital informationcan be used to assemble a final image. In some CMOS sensors, at leastsome of the readout electronics 220 can be embedded in the individualpixels of the image sensor. It should also be understood that the CMOSimage sensor 210 of FIG. 2b can include other components that areomitted from the figure for purpose of clarity.

This architecture of the CMOS sensor 210 allows each of its pixels 212(or each row/column of pixels) to be turned on/off independently and thesignals from the pixels (or rows/columns of pixels) to be read outsequentially. In operation, the clock and timing control module 216 cansend out timing signals to the individual pixels (or individualrows/columns of pixels) to control the timing of their exposure. As soonas one pixel is exposed, its value can be read out. There is virtuallyno delay between the exposure and the readout. In addition, theshuttering effect with regard to each pixel (or row/column of pixels)can be programmed to occur on a rolling basis across all pixels. Forexample, when the pixel value of the first pixel 211 is being read out,the second pixel 212 can be finishing its capturing process. Similarly,when the pixel value of the second pixel 212 is being read out, thethird pixel 213 can be capturing light. Similarly, if a row of pixels isexposed and readout together, the next row can begin capturing lightbefore the previous row finishes its readout. Essentially, every pixel(or row of pixels) is read out a bit later than every other pixel (orthe next row of pixels). Every pixel (or row) readout can be skewedrelative to its neighbor by a fraction of, for example, a microsecond.Thus, there is almost always at least one pixel being exposed and thereis almost no delay between exposure and readout for any individualpixel. This allows cameras with rolling shutter imagers to reduce, forat least some of the pixels, the delay between the exposure of the pixeland analysis of the information read out from the pixel. The reductioncan be especially significant for pixels that are exposed and read outlast compared to if they were processed by a global shutter imager.

However, because the rolling shutter staggers the exposure time for theindividual pixels (or individual rows/columns of pixels), theinformation captured by different pixels will be captured at differentmoments in time. If there are relative movements between objects in thescene being captured and the camera, the result image can havenoticeable motion artifacts such as wobble, skew, smear, etc. due to thestaggered exposure of the different pixels. This is one of the drawbacksof a rolling shutter imager. In addition, one or more keypoints may bedistorted beyond recognition as a result of not all the pixels in akeypoint is exposed at the same time. The distorted keypoints may nolonger be able to identify the corresponding features in a series ofconsecutive frames, thus affecting the camera's ability to track its ownmovement or the movement of an object being captured.

In short, both of the above-discussed shutter mechanisms havesignificant drawbacks: the global shutter imagers can cause delaysbetween pixel exposure and analysis and the rolling shutter imagers cancause undesirable effects (e.g., distortions) to the resultant images.To minimize these shortcomings, the following embodiments disclose imagesensors with a third type of shutter mechanism, referred to herein as a“semi-global shutter.” A semi-global shutter can reduce the delaysbetween pixel exposure and analysis while reducing certain forms ofundesirable distortions.

Specifically, image sensors with semi-global shutters can divide (orgroup) its two-dimensional pixel array into multiple pixel blocks, eachblock including multiple pixels in a region of the image area. Asemi-global shutter can allow all pixels in the same block to be exposedsimultaneously and read out as a group. As such, semi-global shuttersmay be able to capture images free of motion artifacts, at least withineach pixel block. The blocks can be exposed and readout on a rollingbasis. That is, while data from one block of pixels is read out andprocessed, the next block of pixels can be exposed. This can allow theblocks to be exposed in an overlapping or sequential manner, which cansignificantly reduce or eliminate the delay between consecutiveexposures that a global shutter imager typically suffer. This canprovide speed advantages.

In addition, semi-global shutters can reduce the amount of motionartifacts that can often be associated with images taken with rollingshutter imagers. In various embodiments, this can be achieved bydividing the pixels into the optimal number of pixel blocks accordingthe requirements and/or intended usage of the camera. As discussedabove, a rolling shutter is typically programmed to expose and read outone pixel or one row/column of pixels at a time. For an image sensorwith a large number of pixels (or rows of pixels), the difference in theexposure time of each pixel or row of pixels caused by the rollingexposure can be substantial, resulting in easily-noticeable motionartifacts in the final images. A semi-global shutter can roll throughblocks of multiple rows/columns of pixels at a time. As an example, ifeach block has 10 rows, the amount of potential motion artifacts couldeffectively be reduced by a factor of about ten compared to a rollingshutter that rolls through the pixels one row at a time. Ideally, thepixel blocks can be defined to achieve a balance between frame rate andimage quality for any particular camera with a semi-global shutterimages.

FIG. 3 is a block diagram illustrating the exemplary components of animage sensor 300 with a semi-global shutter. The image area of thesensor 300 can be divided into 9 pixel blocks in a 3×3 layout. Forexample, the top row can include pixel blocks 301, 302, 303. Each blockcan include multiple pixels. An enlarged view of block 303 shows thatblock 303 includes a 4×4 array of pixels. In this embodiment, each blockcan include the same number of pixels arranged in the same formation.However, the disclosure is not so limited and in other embodiments theblocks can include different numbers and/or arrangements of pixels.Every pixel 312 in the blocks can be a sensor element (i.e., photosite)capable of converting light into electronic signals.

A number of block signal lines (collectively 304) can transmit timingsignals from a clock and timing control module 308 to each of the ninepixel blocks. The timing signal can control when each pixel block isexposed to light and also when the electronic signals from the pixelblocks are read out. Although only three block signal lines are shown inFIG. 3, it should be understood that every pixel block may be separatelyconnected to the clock and timing control module 308 and can betriggered independently. A number of output lines 306 can transmit theelectronic signals from the pixels in each pixel block to readoutelectronics 316 for processing. Only three vertical lines are marked asoutput lines 306 in FIG. 3 for clarity purposes. However, it should beunderstood that every pixel block may be separately connected to thereadout electronics 316 and the signals from each pixel block can beread out in parallel. The readout electronics 316 in this embodiment canbe designed to process signals received from each of the nine pixelblocks in parallel. As illustrated, the readout electronics 316 caninclude nine separate submodules each responsible for processing signalsfrom one of the nine pixel blocks. For example, submodules 321, 322, 323can readout signals from pixel blocks 301, 302, 303 respectively. Eachsubmodule can include, for example, an amplifier and A/D converter foramplifying and converting the signals into a digital format (e.g.,digital numbers), respectively. The digital information can be processedto assemble a final image. It should also be understood that thesemi-global shutter imager 300 of FIG. 3 can include other componentsthat are omitted from the figure for clarity purposes. The exemplaryarrangement schematically shown in FIG. 3 may be expanded for anysuitable number of pixel blocks and their respective submodules.

In operation, the semi-global shutter can be programmed to expose thepixel blocks on a rolling basis. For example, within the same pixelblock, all the pixels can be exposed simultaneously and the signals fromeach pixel in the pixel block can be read out one pixel at a time andtransferred over one of the output lines to a corresponding submodule inthe readout electronics 316. Each pixel block may not be exposed againuntil the readout is completed. However, while the first pixel block 301is being read out, a second pixel block 302 can be exposed. Similarly,while the signals from the pixels in the second pixel block 302 arebeing read out, a third pixel block 303 can be exposed. This overlappingin exposure and readout of the different blocks can minimize oreliminate the delays that typically occur between exposures in a globalshutter imager.

FIG. 4 illustrates an exemplary timeline of the exposures (top) andreadouts (bottom) of three of the pixel blocks 301, 302, 303 in thesemi-global shutter imager 300 of FIG. 3. As illustrated, the exposureand readout of each block 301, 302, 303 can be sequential, but thetriggering of successive blocks can be overlapping. Each pixel block canbe exposed slightly after its predecessor, with the exposure delayallowing for readout of one or more previously-exposed pixel blocksbefore the current pixel block completes its exposure. As an example,exposure of pixel block 302 in the sequence may be triggered before thereadout time for the previous pixel block (e.g., pixel block 301) can becompleted. Similarly, the exposure of block pixel 303 may be triggeredbefore the readout time for block 302 can be completed. This canincrease the use of the output digital transfer bandwidth from theimager. When all the pixel blocks (including the six not shown in thetimeline of FIG. 4) complete a cycle of exposure and readout. The firstblock (i.e., block 301) can be exposed again without any delay.

Alternatively, the pixel blocks can be triggered sequentially, in whicheach block can begin its exposure only after the prior pixel block hascompleted its exposure. For example, this can be suitable for computervision applications, as blurring effects from motion may not includeidentical sub-windows of integration time. Regardless of whetheroverlapping or sequential triggering is implemented, the pixel blockscan be read out immediately after their exposure time is complete.

Because there is less delay between the exposures of a pixel block andthe analysis of the information captured by the pixels in the pixelblock, the information generated by a semi-global shutter imager can bemore accurate in reflecting the location and/or movement of the cameraand/or scene being captured than a global-shutter imager of the samepixel resolution, which would require the entire image to be readoutbefore information from the pixels can be analyzed. One potential delayin a semi-global shutter imager may occur when the signals fromindividual pixels in the same block are read out. However, this delaycan be significantly shorter than the delay in a comparable globalshutter imager. For example, if the delay on a global shutter imager is15 milliseconds for the pixels read out last, the delay on a semi-globalshutter imager for the same pixels (or any of the last pixels in eachblock) is only 1.5 milliseconds if the semi-global shutter imager isdivided into ten pixel blocks of the same size. Accordingly, the imagescaptured with semi-global shutter imagers may be superior for machinereadable images such as those used for tracking keypoints in an image.

Although both semi-global shutters and rolling shutters can capture animage by scanning across the scene rather than taking a snapshot of theentire scene, semi-global shutter can achieve better image quality thana rolling shutter when there is relative movement between the camera andthe scene for some applications. This is because the number of pixelblocks in a semi-global shutter imager can be much lower than the numberof pixels or rows in a rolling shutter imager. By dividing the pixelsinto fewer pixel blocks, the motion artifacts in the final image can beconfined. The only regions in an image that may be affected by thesequential exposure of the pixel blocks are the boundaries between theblocks. In contrast, motion artifacts can appear anywhere on an imagetaken by a camera with rolling shutter imager because the rollingshutter rolls through the pixels one row at a time. However, theseimages may be suitable for machine readable images such as those usedfor tracking keypoints in an image.

As an example, a 9×9 pixel array of a semi-global shutter imager can bedivided into three pixel blocks each including three rows of pixels,resulting in two boundaries between the three pixel blocks. If the same9×9 pixel array is read out row by row using a rolling shutter, thereare eight boundaries between the nine rows. That corresponds tosignificantly more areas that may be affected by motion artifacts in theimages taken with the rolling shutter imager than those taken with thesemi-global shutter imager. In addition, the fewer boundaries in thesemi-global shutter can also mean that fewer keypoints would getdistorted beyond recognition when the shutter rolls through the pixelblocks.

A semi-global shutter can divide the image array into any suitablenumber of pixel blocks and each pixel block can have any suitable numberof pixels. In various embodiments, any arrangement of pixel blocks,including ones of non-equal size, non-compact, or non-contiguousconfigurations, is possible. Preferably, the number of pixel blocks canbe set to both achieve a shorter delay between pixel exposure andanalysis than a comparable global shutter imager and be less prone tocertain forms of distortions in the result images than a comparablerolling shutter imager. For example, one might wish to divide a1000×1000 pixel image area of a semi-global shutter imager into 10blocks (e.g., each a 100×1000 block). It can reduce the delay betweenpixel exposure and analysis, for at least some of the pixels, by 10times compared to a global shutter imager with the same 1000×1000 pixelimage area. Additionally, the rolling shutter effect (e.g., motionartifacts) can only be an issue at the boundaries between the pixelblocks rather than throughout the image area as would be the case in arolling shutter imager.

For cameras that require a short delay between pixel exposure andanalysis, it would be ideal to have a semi-global shutter imager havingan image area divided into a large number of pixel blocks, eachincluding fewer pixels. In contrast, for cameras that favor imagequality (e.g., minimizing certain forms of distortion) over shorterperiods of delay, a semi-global shutter imager with a small number ofpixel blocks can be more desirable.

A semi-global imager may include pixel blocks of any suitable geometricshape and arrangement. While FIG. 3 illustrates one embodiment in whichthe exemplary blocks are squares of the same size, FIGS. 5a-5cillustrate other exemplary divisions of pixel blocks suitable forvarious semi-global shutter imagers. In particular, FIG. 5a illustratesan image area divided horizontally into five (or N number of)rectangular pixel blocks 510, each including the same number of rows ofpixels. FIG. 5b illustrates an image area divided vertically into five(or M number of) pixel blocks 520, each having the same number ofcolumns of pixels. FIG. 5c illustrates a configuration of five pixelblocks 530 that are of different shapes and/or sizes. As discussedabove, the number and/or geometric shape of the pixel blocks can beoptimized so that the areas between the pixel blocks are kept to aminimum. This can provide significant advantage in image quality for asemi-global shutter imager than a rolling shutter imager.

In one embodiment, instead of dividing the image area of an relativelylarge resolution image sensor into multiple pixel blocks (as shown inFIG. 3 for example), a semi-global shutter can be implemented byassembling multiple relatively low-resolution imagers in the sameconfiguration (e.g., each block in FIG. 3 substituted by a standalonelow resolution imager). These relatively low-resolution imagers can havetheir triggers staggered in time and controlled by a central controller.Each imager can individually operate at a pixel clock rate such that thetransmission per pixel block is inversely a function of the size of theimager. In one embodiment, when cutting the wafer for the low-resolutionimagers, the dies for the individual imagers may not be separated by thecutting.

It should be understood that, the application of the present disclosureis not limited to the above-mentioned embodiments. It will be possiblefor a person skilled in the art to make modifications or replacementsaccording to the above description, all of those modifications orreplacements shall all fall within the scope of the appended claims ofthe present disclosure.

What is claimed is:
 1. An image sensor comprising: a two-dimensional pixel array divided into a plurality of blocks, each of the plurality of blocks comprising pixels arranged in at least two different rows and two different columns, and a shutter that exposes the plurality of blocks sequentially, with all pixels in each block being exposed synchronously.
 2. The image sensor of claim 1, wherein the two-dimensional pixel array includes M rows of pixels, M no less than 100, and wherein a height of each block is at least one twentieth of a combined height of M rows but no more than one fifth of the combined height of M rows.
 3. The image sensor of claim 1, wherein the two-dimensional pixel array includes N columns, N no less than 100, and a width of each block is at least one twentieth of a combined width of N columns of pixels but no more than one fifth of the combined width of N columns.
 4. The image sensor of claim 1, wherein at least two of the blocks include different number of pixels.
 5. The image sensor of claim 1, wherein at least two of the blocks have different geometric shapes.
 6. The image sensor of claim 1, wherein each of the plurality of blocks includes a same number of pixels.
 7. The image sensor of claim 1, comprising: a timing control module that transmits a timing signal to each of the blocks, the timing signal initiating a sequence of exposures of the blocks.
 8. The image sensor of claim 1, comprising: separate readout electronics for each of the plurality of blocks, the readout electronics capable of receiving and processing signals from the pixels in a corresponding block.
 9. The image sensor of claim 8, wherein the readout electronics comprises an amplifier that amplifies the signals and an analog-to-digital converter that converts the signals to digital data.
 10. The image sensor of claim 8, wherein the plurality of blocks include a first block and a second block, wherein the readout electronics reads out the signals from the first block of pixels immediately after an exposure of the first block is completed, and wherein an exposure of the second block begins before the readout of the signals from the first block is completed.
 11. The image sensor of claim 8, wherein the plurality of blocks include a first block and a second block, and wherein there is a delay between the exposure of the second block and the exposure of the first block, the delay being great enough to allow for a readout of the first block before the second block completes its exposure.
 12. The image sensor of claim 1, wherein the shutter is electronically-controlled.
 13. A method of capturing an image with an image sensor, comprising: dividing a two-dimensional pixel array image area of the image sensor into a plurality of blocks, each of the plurality of blocks comprising pixels arranged in at least two different rows and two different columns, and sequentially exposing the plurality of blocks, with all pixels in each block being exposed synchronously.
 14. The method of claim 13, wherein the two-dimensional pixel array includes M rows of pixels, M no less than 100, and wherein a height of each block is at least one twentieth of a combined height of M rows of pixels, but no more than one fifth of the combined height of M rows.
 15. The method of claim 13, wherein the two-dimensional pixel array includes N columns, N no less than 100, and a width of each block is at least one twentieth of a combined width of N columns of pixels but no more than one fifth of the combined width of N columns.
 16. The method of claim 13, comprising: transmitting a timing signal to each of the blocks, the timing signal initiating a sequence of exposures of the blocks.
 17. The method of claim 13, comprising: reading out signals from at least a first block and a second block of the plurality of blocks sequentially.
 18. The method of claim 17, wherein the signals from the first block of pixels is read out immediately after an exposure of the first block is completed, and wherein an exposure of the second block begins before the readout of the signals from the first block is completed.
 19. The method of claim 17, wherein there is a delay between the exposure of the second block and the exposure of the first block, the delay being only great enough to allow for a readout of the first block before the second block completes its exposure.
 20. A digital camera comprising: an image sensor comprising a two-dimensional pixel array divided into a plurality of blocks, each of the plurality of blocks comprising pixels arranged in at least two different rows and two different columns; a lens that directs light to the image sensor; a shutter that exposes the plurality of blocks sequentially, with all pixels in each block being exposed synchronously; a timing control module that controls the timing of a sequence of exposures of the blocks; separate readout electronics for each of the plurality of blocks, the readout electronics capable of receiving and processing signals from the pixels in a corresponding block; and a camera ASIC that assembles an image from an output of the readout electronics. 